TAS5705PAPR datasheet(38/71 Pages) TI

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AlphaFilterStructure

B0265-01

Energy

Filter

a w

T,K,O

DRC1

DRC2

0x3A

0x40,0x41,0x42

0x3B/0x3C

0x3E/0x3F

0x43,0x44,0x45

0x3D

Compression

Control

Attack

and

Decay

Filters

AudioInput

DRCCoefficient

NOTE:

=1 – a

AM Tuner Interference Management

TAS5705

SLOS549 – JUNE 2008 ...................................................................................................................................................................................................... www.ti.com

Figure 44. DRC Structure

Digital amplifiers produce AM interference by radio energy emissions near the digital amplifier switching rate and

the harmonics of that switching rate. The digital amplifier emits an interference spectrum in the AM band that is

centered on the second though sixth harmonics of the digital amplifier switching frequency. Because the digital

amplifier switching rate is a multiple of the input data sample rate, the interference frequencies can be changed

by changing the sample rate.

Anatomy of a Receiver

AM receivers are composed of six sections as shown in Figure

The radio-frequency (RF) section has a variable tuner that preselects the frequencies to be received. The RF

section provides only enough filtering to significantly attenuate signals that are significantly above or below the

desired tuned frequency. The RF-section gain is controlled by the AGC to compensate for variations in the signal

strength.

The mixer, variable-frequency oscillator (VFO), and intermediate frequency (IF) sections perform the bulk of the

receiver tuning. In home receivers, the variable oscillator is adjusted so that it is a constant 455 kHz higher than

the desired tuned frequency. The output of the mixer consists of four frequencies, RF, VFO, RF + VFO and RF –

VFO. These signals are then passed into the IF section, where sharp discriminating filters to accept only the

constant 455-kHz (RF – VFO) signal. The resulting signal is amplified and then passed to the detector section,

where it is transformed into an audio signal.

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